1. Technical Field
The present invention relates to an endoscope apparatus having a front end portion in which an imaging element is built, and a method for releasing heat generated by the imaging element.
2. Related Art
When an electronic apparatus is assembled, generally, a flexible substrate is used for enhancing the degree of freedom of the assembling. When such a flexible substrate is used, the flexible substrate is bent at any place so that the electronic apparatus is easily assembled in a narrow site easily.
For example, in an electronic endoscope apparatus described in JP-A-2010-69217, an imaging element is mounted on a flexible substrate so that the imaging element can be assembled in a narrow endoscope front end portion easily.
A structure which releases heat well is required for receiving an imaging element, a circuit for driving the imaging element, or the like in a narrow endoscope front end portion. The imaging element may be a heat generating element. In the electronic endoscope apparatus described in JP-A-2010-69217, a part of the flexible substrate is used as a heat release portion.
In recent electronic endoscope apparatuses, the diameter of a front end portion to be inserted into a subject is about 1 cm, and the diameter has a tendency to decrease. Further, there is a request to shorten the length of the front end portion. In addition, there is a tendency to increase the number of pixels in an imaging element and to increase the driving frequency to improve the image quality of a captured image.
The imaging element is a heat generating component. With increase in the number of pixels in the imaging element, the heat value of the imaging element increases. In addition, when the imaging element is driven at a high speed, the heat value of the driving circuit also increases. It is therefore necessary to build a structure with further improved heat release characteristic in an electronic endoscope apparatus which receives the imaging element in the endoscope front end portion. However, when the endoscope front end portion is reduced in diameter and shortened in length, the imaging element and a flexible substrate mounted with the imaging element needs to be miniaturized. Thus, it is difficult to secure a path to release heat generated by the imaging element.
JP-A-2002-344095 has disclosed a technique in which a graphite layer with high heat conductivity is laminated on the surface of a resin layer of a flexible substrate in order to enhance heat release performance of the flexible substrate. However, when the graphite layer with high heat conductivity is laminated on the flexible substrate, the thickness of the flexible substrate itself increases so that the flexible substrate cannot be received in a narrow site.
In addition, an electronic endoscope apparatus according to JP-A-2003-10111 has used a structure in which a large GND terminal region of a signal cable connected to an imaging element is secured so that heat generated by the imaging element can be released to the signal cable through the GND terminal. However, only with the heat release structure according to JP-A-2003-10111, there is a fear that satisfactory heat release characteristic cannot be obtained when an imaging element which has a larger number of pixels and is driven at a higher speed is received in an endoscope front end portion having a smaller diameter.